Seven dogs from Cape Cod and 8 dogs from Martha's Vineyard were diagnosed clinically and serologically as having Rocky Mountain spotted fever; R. rickettsii was isolated from 3 of the dogs. R. montana, however, has been isolated from approximately 1% of the ticks examined. R. montana appears to have antigens in common with R. rickettsii as evidenced by antibody determinations made from the sera of sick and asymptomatic but sera-positive dogs. Sera from asymptomatic dogs previously shown to be reactive with R. rickettsii were reactive to the same or to a higher degree with R. montana.